In a moncoque automotive body structure, various body panel members are stamp formed from thin steel plate, and are joined by spot welding or the like so as to form the final body structure. The floor panel assembly that forms the floor of the body is typically provided with reinforcements such as longitudinal floor frames and floor cross members on the upper and lower surfaces thererof so as to ensure the mechanical strength and rigidity that are required to support the weight of the seats and vehicle occupants.
When providing a longitudinal floor frame to the lower surface of the floor panel, it is desirable that the longitudinal floor frame has a substantially constant cross section over the entire length thereof to minimize vibrations and noises, prevent stress concentration and ensure a favorable distribution of crash impact energy.
Electropaining is commonly used for painting automotive bodies. This process involves the dipping of the body into water containing particles of paint suspended therein, and the body is required to have an arrangement for facilitating the draining of the water trapped in the body when the body is lifted out of the water. For this purpose, an automotive floor panel is typically provided with a recessed portion in a longitudinally middle portion thereof and draining holes are formed in low parts of such a recessed portion. A longitudinal floor frame for such a floor panel is typically curved so as to extend along the curved contour of the lower surface of the floor panel, and is provided with a substantially uniform height over the entire length thereof. Such an example can be found in Japanese patent laid open publication No. 2003-252251.
FIG. 6a shows a conventional arrangement in which a longitudinal floor frame 31 is attached to the lower surface of a floor panel which is provided with a recess 22 defining an upper concave surface and a lower convex surface. The longitudinal floor frame 31 is provided with a curved section 31a which is curved so as to conform to the lower convex surface. The curved section 31a reduces the height Z of the lowest part of the body from the ground level, and this impairs the capability of the automobile to travel in a rugged terrain or creates the need to raise the height of other parts of the automobile body.
This problem can be overcome by locally reducing the height of the longitudinal floor frame 31 as indicated by numeral 31b in FIG. 6b so as to accommodate the lower convex surface of the floor panel while keeping the lower surface of the longitudinal floor frame 31 flat or horizontal over the entire length thereof. However, this part 31b which is reduced in height creates a weak portion 31c having a reduced cross section. Also, stress tends to concentrate in such a portion lacking in uniformity.